Foam Core Sandwich Composites Research Articles

Shift in failure modes in foam core sandwich composites subject to repeated slamming on water

AbstractA test program designed and carried out to mimic the repeated impact of the bow section of fast-moving small boats on the ocean surface provided some unique observations in terms of failure mode transition. Damage progression and modes of failure were evaluated for two types of sandwich composites with comparable global strength and stiffness but different foam density and facesheet strength. Testing was performed on flat rectangular specimens that contained symmetric semi-elliptical edge flaws produced near the end of the specimen held by the rotating cam. Type 1 specimens (softer core/stronger facesheet) consistently failed by interface and through-the-thickness core shear, independent of the flaw size. In contrast, a gradual decrease in flaw size in Type 2 specimens (denser core/weaker facesheet) produced a striking transition in the mode of failure from local buckling in the vicinity of the flaw site along with exponentially increasing lifetime, to interface shear failure at the free end accompanied by a dramatic drop in lifetime. The lifetime of Type 2 specimens was more than two orders of magnitude greater than that of Type 1 specimens.

Numerical Simulation of Impact Responses on Through-thickness Stitched Foam Core Sandwich Composite

This paper was based on the explicit finite element codes to predict the impact behavior of through-thickness stitched foam core sandwich composites. It is proposed that the extent of the impact damage can be characterized by the token parameters of cracking width, penetration depth and damage angle; and observations made during the simulative analysis with such damage parameters. The results show that the same tendencies and characteristics are shown on the numerical and test results of impact force-displacement plots, and a good agreement is also obtained in damage parameters. In comparing the unstitched types, the through-thickness stitched sandwiches are optimal for both the peak loads shown on the numerical plots at 25.0 J; and demonstrate the fewer extent of impact damage with a 63.5 and 6.0 % decreasing to the cracking width and penetration depth respectively, and where a 52.0 % increasing to the damage angle.

Static behavior of three-dimensional ıntegrated core sandwich composites subjected to three-point bending

In the current study, the effect of the thickness and the foam density in three-dimensional integrated woven sandwich composites on quasi-static mechanical properties under three-point bending was investigated. Bending modulus and core shear modulus were determined by subjecting the samples, which were cut with varying span lengths according to their core thicknesses, to three-point bending test. Obtained results were optimized by taking core thickness, foam density and panel weights into consideration. Damages that occurred on the tested samples were reported. When compared to conventional foam core sandwich composites, it was found that three-dimensional integrated sandwich composites have superior mechanical properties and due to the fact that the pile yarns in the core and the foam support each other, contrary to conventional sandwich composites no catastrophic core breakage occurs under load, thus the load bearing capacity of the structure is sustained.

Quasi-static behavior of three-dimensional integrated core sandwich composites under compression loading

In the current study, the effect of the thickness and the foam density in three-dimensional integrated woven sandwich composites on quasi-static properties was investigated. For this purpose, produced samples were subjected to uniaxial flatwise compression tests and their compression strength and moduli were determined. Obtained results were optimized by taking core thickness, foam density and panel weights into consideration. Damages that occurred on the tested samples were reported. When compared to conventional foam core sandwich composites, it was found that three-dimensional integrated sandwich composites have better compression properties and due to the fact that the pile yarns in the core and the foam support each other.

The Sensitivity of the Foam-Core Parameters under Impact Loading

This study presents the numerical investigation of the low-velocity impact for the foam-cored sandwich composites. Firstly, the proposed FEA model is validated by comparing the results between simulation and test. The user subroutine VUMAT and the crushable foam model are chosen to describe the damage of the face sheets and the characteristics of the foam material, respectively. The detailed damage process of the sheets and the foam is clearly shown. The sensitivity of seven parameters related to foam-core material are studied. It is shown that the yield strength, the fracture strain and the fracture displacement have significant effects on the impact-resistance of the foam-cored sandwich composites.

Mode III Fracture Study of Foam Core Composite Sandwich Beams

The present paper is concerned with the problem of mode III fracture in foam core composite sandwich beams. Three-dimensional linear-elastic finite element simulations were performed of sandwich beams with a crack in the mid-plane of the core. The load of the sandwich beam was chosen such that the cross-sectional bending moment in the arms at the crack front was reduced to zero in order to minimize the mode II fracture. Within the finite element analysis, the fracture was studied in terms of the strain energy release rate using the virtual crack closure technique. The calculations revealed that pure mode III loading conditions were induced along the crack front, except a small zone near the free edges of the sandwich beam cross-section, where mode II component of the strain energy release rate was also found. The influence of the sandwich core material on the mode III fracture was analyzed. For this purpose, three sandwich beam configurations were considered (it was assumed that the sandwich core was made by three different rigid cellular foams). It was found that the strain energy release rate decreases with increase of the sandwich core density.

Compression strength of stitched foam-core sandwich composites with impact induced damage

Compression-after-impact responses of sandwich structures composed of stitched foam core and woven face sheets have been investigated experimentally and numerically. In the impact experiment, the force-time curves are recorded and analyzed to study the impact response of stitched foam core sandwich structures. In the compression-after-impact test, the damage mechanism of stitched sandwich structures during compression loading is revealed. The compression cracks start from the impact-induced damage location and propagate perpendicular to the compression direction. The final failure of the structure is controlled by the crush of the woven skin in the upper panel. For the relatively densely stitched foam sandwich, more fiber resin columns embed in the stitched samples decrease the impact-induced damage in upper panel when subject to impact load and increase the stiffness and strength of the foam in the compression direction during the compression-after-impact test, causing an increase in compression strength of relatively densely stitched sandwich. The relationship of compression-after-impact strength with stitching density appears linear when the impact energy is small, since there is no perforation or penetration. In numerical simulation, multi-scale approach has been developed on evaluating the compression-after-impact strength of sandwich structures. The classical theory of homogenization is adapted and used by treating the foam strengthened by the glass fiber resin column as orthotropic equivalent core material whose elastic properties depended on each component and their volume participation. Then, a nonlinear finite element model using this approach is proposed. Good agreements between finite element model and experiment are found.

Face/core debond fatigue crack growth characterization using the sandwich mixed mode bending specimen

Face/core fatigue crack growth in foam-cored sandwich composites is examined using the mixed mode bending (MMB) test method. The mixed mode loading at the debond crack tip is controlled by changing the load application point in the MMB test fixture. Sandwich specimens were manufactured using H45 and H100 PVC foam cores and E-glass/polyester face sheets. All specimens were pre-cracked in order to define a sharp crack front. The static debond fracture toughness for each material configuration was measured at different mode-mixity phase angles. Fatigue tests were performed at 80% of the static critical load, at load ratios of R=0.1 and 0.2. The crack length was determined during fatigue testing using the analytical compliance expression and verified by visual measurements. Fatigue crack growth results revealed higher crack growth rates for mode I dominated loading. For specimens with H45 core, the crack grew just below the face/core interface on the core side for all mode-mixities, whereas for specimens with H100 core, the crack propagated in the core or in the face laminate depending on the mode-mixity at the debond crack tip.

Damage Behavior of Foam Core Sandwich Composites Subjected to Quasi-Static Indentation Testing

Damage behavior of foam core sandwich composites subjected to quasi-static indentation testing was investigated detailedly. Force-displacement curves and the cross-sections of damaged sandwich panels were analyzed. Effects of core thickness, face-sheet thickness and toughness of face-sheet materials were discussed by comparison with different specimens subjected to quasi-static indentation testing. Results showed that damage behavior of sandwich composites had identical feature. Collapse of foam core initially appeared, and then foam core generated crack. Delamination and fiber breakage of upper face-sheet occurred. Finally, the foam core and the face-sheet were disengaged. For the specimens with thick foam cores, the foam crack extended in longitudinal direction was caused. However, for the specimens with thin foam cores, the foam cracked in transverse direction. After face-sheets were toughened with PAEK, the damage resistance and the maximum force of the sandwich panels increased. Also, the shear angle of foam crack changed to be lager. Compared with the penetrated damage behavior of sandwich panels with thin face-sheets, the upper face-sheets of the former the specimens with thick face-sheets bulged partially and could not be penetrated.

Experimental impact damage study of a z-pinned foam core sandwich composite

This paper assesses the impact damage and post-impact compression properties of a foam core sandwich composite reinforced with through-thickness z-pins. Low-speed flat-wise compression tests performed on the sandwich composite revealed that z-pins improved the elastic modulus, crush strength and absorbed energy capacity (by 260–300%). However, these property improvements do not necessarily translate into a reduction in the amount of damage suffered by the z-pinned sandwich composite under localised (point) impact loading. There was no reduction to the impact damage area or an improvement to the post-impact compression properties of the z-pinned sandwich composite at low-impact energies (when damage was confined to the impacted face skin). Z-pins were only marginally effective at reducing the impact damage when the impact energy was high enough to cause core crushing. Z-pins absorbed high-impact energy via splitting, microbuckling and fragmentation during core crushing which reduced slightly the amount of impact damage to the sandwich composite. However, this did not cause a significant improvement to the post-impact compressive stiffness and strength for most energy levels.

Pressure pulse response of composite sandwich panels with plastic core damping

An analytical technique is presented for obtaining the large amplitude, damped response of a foam-core composite sandwich panel subjected to uniform pressure pulse loading. The predicted panel response was in agreement with finite element analysis, and it was used to examine the blast resistance of panels with various foam cores. Although blast resistance generally increased with the increasing core density, panels with the densest cores exhibited no energy absorption before panel failure. The panel with the PVC H200 foam also sustained higher failure pressure per unit areal weight density than the denser panels with Klegecell R300 and HCP100 foams.

Natural Cork Agglomerate Employed as an Environmentally Friendly Solution for Quiet Sandwich Composites

Carbon fiber-synthetic foam core sandwich composites are widely used for many structural applications due to their superior mechanical performance and low weight. Unfortunately these structures typically have very poor acoustic performance. There is increasingly growing demand in mitigating this noise issue in sandwich composite structures. This study shows that marrying carbon fiber composites with natural cork in a sandwich structure provides a synergistic effect yielding a noise-free sandwich composite structure without the sacrifice of mechanical performance or weight. Moreover the cork-core sandwich composites boast a 250% improvement in damping performance, providing increased durability and lifetime operation. Additionally as the world seeks environmentally friendly materials, the harvesting of cork is a natural, renewable process which reduces subsequent carbon footprints. Such a transition from synthetic foam cores to natural cork cores could provide unprecedented improvements in acoustic and vibrational performance in applications such as aircraft cabins or wind turbine blades.

Blast Performance of Marine Foam Core Sandwich Composites at Extreme Temperatures

The dynamic behavior of sandwich composites for marine applications, made of E-glass Vinyl Ester skin and Corecell™ M100 foam core material, was studied at different temperatures using a shock tube apparatus. High-speed photography coupled with the optical technique of Digital Image Correlation was used to record the real time deformation of the specimen during shock wave loading. Sandwich composites under a high temperature environment demonstrated a significant amount of fiber breakage, fiber-matrix delamination as well as core compression when subjected to a shock wave loading. When subjected to low temperature environment and shock loading, the core material showed brittle behavior resulting in core-cracking in the sandwich composite. The dynamic response and failure mechanisms demonstrated by the sandwich composites at these temperatures were in correspondence with the high strain rate behavior of the individual constituents.

Comparison with Low-Velocity Impact and Quasi-staticIndentation Testing of Foam Core Sandwich Composites

The need for quasi-static indentation test method for modeling low-velocity foreign object impact events would prove to be very beneficial to researchers. In order to examine whether it is feasible, series of quasi-static indentation and low-velocity impact tests were carried out and compared. An analysis of the relationships between impact energy (or quasi-static indentation force) and damage area, dent depth indicates clearly that dent depth was selected as the damage parameter to set up damage relationship between the two tests. The knee points of dent depth appearing in the two tests curves were very close. The variation tendency of the dent depth, the process curves and the cross sectional damage views of the two tests were in similarity. Results show that no distinct differences could be seen between low-velocity impact and quasi-static indentation testing, indicating that quasi-static indentation testing can be used to represent low-velocity impact testing.

Damage Identification After Impact in Sandwich Composites Through Embedded Fiber Bragg Sensors

Based on the full-spectral response of fiber Bragg grating sensors, embedded at the facesheet-core interface, we identify the progression of failure modes in foam core sandwich composites during multiple, low-velocity impacts. By considering the characteristic shape of the reflected spectrum from the FBG sensor in the post-impact, residual strain state, it is shown that we can classify the extent of damage into one of three states. Unlike the previous FBG peak wavelength measurements; this identification does not require the full strain history to identify the current state of damage in the composite. The disparate material properties between the facesheet and core materials, which create significant challenges for conventional non-destructive evaluation methods, enhance the damage detection through large deformations in the core at the impact location with sharp strain gradients.

Stress Field Evolution under Mechanically Simulated Hull Slamming Conditions

The effect of repeated loading from mechanically simulated hull slamming on foam core sandwich composites was investigated utilizing a novel technique that simultaneously measured temperature and displacement while cyclic loading occurred. Thermoelastic Stress Analysis (TSA) and Digital Image Correlation (DIC) techniques were combined using a single infrared camera for characterization of the foam core. Improved stress fields with TSA results were found through deformation compensation. Initial work approximating hull slamming conditions mechanically utilizing a custom device were performed. Mechanically loading offers several benefits over water impact investigations, including easy access to the sample during the slamming event, an unobstructed optical path, and accelerated testing. Evolving stress fields under long-duration, repeated simulated hull slamming loading were observed around a growing delamination crack between the foam core and skin.

The interfacial fracture behavior of foam core composite sandwich structures by a viscoelastic cohesive model

A sandwich beam model consisting of two face sheets and a foam core bonded by a viscoelastic adhesive layer is considered in order to investigate interfacial fracture behavior. Firstly, a cohesive zone model in conjunction with a Maxwell element in parallel, or with a Kelvin element in series, respectively, is employed to describe the characteristics of viscoelasticity for the adhesive layer. The models can be implemented into the implicit finite element code. Next, the parametric study shows that the influences of loading rates on the cohesive zone energy and strength are quite different for different models. Finally, a sandwich double cantilever beam model is adopted to simulate the interface crack growth between the face sheet and core. Numerical examples are presented for various loading rates to demonstrate the efficacy of the rate-dependent cohesive models.

Theoretical Prediction of the Stiffness and Failure Strength of Stitched Foam-Core Sandwich Composites

Based on the microstructure, a novel theoretical method was proposed to predict the equivalent stiffness and failure strength for stitched foam-core sandwich composites. Experimental studies on stiffness and failure strength were also carried out. Considering the different thickness increment of the laminated face-panels for different stitching density and the influence of the stitching angle to the width of the resin pocket zone, a modified fiber distortion model was developed to evaluate the stiffness of stitching laminated face-panels by a combined series-parallel model. As for the prediction of failure strength, the composite face-panels were analyzed by the classical laminate theory and the local wrinkling failure of the face panel was also considered. A bridging model was adopted to analyze the stress of the stitched foam-core, and the failure strengths were predicted with proper failure criteria for different constitutive materials. The equivalent stiffness and failure strength of stitched foam-core sandwich composite panels were obtained under fat-wise loading, edge-wise loading, three point bending and transverse shear loading, respectively. The predicted results showed a good agreement with the experimental results and the finite element results, which demonstrated the validity of the present theoretical method.

Strain energy release rate assessment in mode I delamination of foam core sandwich composites by acoustic emission

In sandwich structures, delamination along the interface between the face sheet and the core is one of the most common failure modes, and it is often referred to as the adherent or adhesive interface in adhesively bonded joints. The three modes of fracture achieved for delamination are modes I, II, and III, in which mode I is the most critical. Delamination depends on stacking sequence, material properties, the production process, and so on, which are measured by mode I interlaminar fracture energy ( GIc). In this study, the double cantilever beam test is used to simulate mode I interlaminar fracture and damage evolution through the acoustic emission (AE) method. Based on AE events and the mechanical properties of sandwich composites, the mode I interlaminar fracture toughness, GIc, is investigated. The influence of in-plane fiber orientation on the fracture behavior of interfaces is studied and compared with other techniques in ASTM. Consequently, the presented method of employing AEs to obtain GIc was determined to be successful.

Studies on VARTM Forming Z-Stitched Foam Core Sandwich Composites

Z-stitched foam core sandwich composite is a newly developed structure with through-thickness load-bearing pillars embedded by a stitching technique. In this paper, large-tow glass fibre stitches and fire-resistant phenolic resin were creatively used to fabricate this kind of sandwich composite, which possessed outstanding properties including fire resistance, debonding resistance and high flatwise compressive strength. Vacuum-assisted resin transfer moulding (VARTM) was conducted for processing. The effects of process conditions on key structural parameters of the z-stitched foam core sandwich composite were investigated, including the dimensions of the pillar, the fibre volume fraction of the facesheet as well as the porosity of the pillar. The results show that macroscopic void defects tend to be formed at the joints between the pillars and the top facesheet if air is trapped in the system. The cross sectional areas of the pillars are all larger than the suture needles due to damage of foam cells in the vicinity during stitching process, which is caused by the brittleness of the phenolic resin. Moreover, the dimensions of the pillar could be affected by the vacuum pressure and the pressure gradient, even resulting in local resin-rich area around pillar. In order to optimize the manufacturing quality of this sandwich composite, a small linear density of stitches and a long enough action time of the vacuum process are both very helpful.